Known devices for controlling the rolling of the vehicle by controlling each portion of a suspension has been developed in order to improve the handling and stability of the vehicle. For example, a known vehicle control device for controlling a distribution of the roll stiffness of the suspension at front wheels and at rear wheels depending on the level of the coefficient of friction on a road is described in JP3163742B2. The roll stiffness distribution is controlled to improve the driving stability of the vehicle upon the smaller coefficient of friction on the road than at the larger coefficient of friction on the road with the vehicle control device described in JP3163742B2. With the construction of the vehicle control device described in JP3163742B2, the coefficient of friction on the road is estimated in accordance with resultant acceleration G when the operational state of the driver should not reduce the horizontal acceleration. This is based on the premise that the reaching of the resultant acceleration G to the peak value means that the tire is reaching approximate the grip limit and the peak value shows the exact coefficient of friction of the road.
With another known device developed for restraining the rolling at the turning operation of the vehicle, the lateral acceleration of the vehicle is applied as one of the control parameters. The lateral acceleration is obtained by a lateral acceleration sensor, by the calculation based on the vehicle speed, the steering angle, and the all the elements of the vehicle, and by the combinations of the lateral acceleration sensor and the calculation. For example, with the device described in JP05(1993)-185815A2, the control gain of the calculated lateral acceleration (i.e., estimated lateral acceleration) calculated from the actual lateral acceleration, the steering angle, and the vehicle speed is adjusted to improve the initial rolling restraining effect at the sudden steering operation and further to maintain the balance with the control corresponding to the actual lateral acceleration at the super sudden steering operation. Further, whether the traveling road surface is at the low coefficient friction is judged based on the power steering hydraulic pressure adjusted in accordance with the steering reaction force using that the steering reaction force relative to the steering assumes smaller at the road with the low coefficient friction. In case it is judged as at the low coefficient friction, the rolling control amount is corrected to reduce, and thus the control can be securely carried out even at the low coefficient friction.
In the meantime, a parameter showing the degree of the force that tire generates relative to the maximum degree of the force that tire generates with the percentage of allowance to the limit (i.e., referred as the lateral grip margin) is applied to control of the steering system and the braking system to consider the improvement of the performance of the vehicle stability control in “Estimation of Grip State Based on Self Aligning Torque and Its Application to Enhance Vehicle Stability” (Muragishi, Yuji et al. 20035105. Spring Academic Lecture on 22 May 2003 by Society of Automotive Engineers of Japan, Inc.) In other words, using the lateral grip margin, the vehicle stability control can be started from the state that the tire is about to reach the limit region, but yet the allowance is still remained. In Muragishi et al., the estimated result of the lateral grip margin is used for the variable control of the overall steering gear ratio. In the present invention, the lateral grip margin is referred as the wheel grip factor.
In the recent development of the electronic technology, the vehicle operation has been computerized. Steer-by-wire-system has been proposed for the steering control. For example, a steer-by-wire system for changing the steering angle in accordance with the movement of an actuator for the steering by a steering member (steering wheel) without mechanically connecting the steering member to vehicle wheels is described in JP2001-191937A2. A steering control device for controlling the steering angle of the rear wheels by the motor actuation is described in JP07(1995)-329808, which is the steer-by-wire system. With the construction described in Muragishi et al, the lateral grip margin (i.e., the wheel grip factor) is obtained in accordance with the operation of an electric power steering device. Although the electric power steering device is adopted according to the present invention, the wheel grip factor is further easily estimated with the steer-by-wire system.
With the construction described in Muragishi et al, although the coefficient of friction on the road can be judged in case the tire reaches the approximate grip limit, the coefficient of friction of the road cannot be judged otherwise, and, for example, it is necessary to have an countermeasure such as using the last value. In other words, when the tire does not reach the approximate grip limit, the estimation precision of the coefficient of friction on the road declines. Thus, it assumes difficult to follow the changes of the state of the road surface, and the effect of the target roll stiffness distribution control may not be sufficiently obtained.
With the construction described in JP05 (1993)-185815, that the traveling road surface has the low coefficient friction is judged in case the power steering hydraulic pressure adjusted in accordance with the steering reaction force assumes equal to or less than a predetermined value in accordance with the steering angle. Notwithstanding, in case, for example, the characteristics of the tire is changed due to the replacement of the tires from the tires for the summer to the tires for the winter, the wear of the tire, and the secular distortion, which affects the changes of the frictional state between the tire and the road surface, the desired characteristic may not be ensured because the judged state of the low coefficient friction of the road is different from the actual state. Using the lateral grip margin described in Muragishi et al., whether the tire is reaching the friction limit region can be accurately estimated.
A need thus exists for a suspension control device which accurately recognizes the frictional state between the road surface and the tire to carry out the appropriate roll control in accordance with the vehicle state.